I'm not following your math where you state that 10 minutes will give the batteries a decent charge. Assuming one is getting 100 amps (amp hours) to the batteries via a decent alternator, it seems that less than 17 amps are actually getting to the battery bank in ten minutes. Not know what size this hypothetical battery bank is, it's hard for me to believe that putting so little power in achieves much of anything.

Ok, maybe I should take that back. I was responding to "floridajakssailor". I agree getting a good charge takes a lot longer, and it all depends on how many batteries you have and what kind of alternator you have. Based on my limited knowledge of batteries it is the beginning of the charging cycle when batteries accept a charge quite rapidly, and that should be enough to keep you going for a little while, with controlled usage. I did also imply that batteries should be maintained with as full a charge as possible. When I'm living on the boat I usually charge my 5 batteries every day or every 2nd day. I try not to let them go below 12V, because as I understand when they go below 12V you are actually hurting them. On the generator my charging system goes thru 3 stages, a full charge cycle takes about 2 and 1/2 to 3 hours. I don't know the math, but I would venture to guess that on an engine alternator it would take just as long if not longer.

My manual for the Universal M3-20 says to put it in gear for 1 to 2 minutes every 2 or 3 hours while charging batteries. This must be a requirement for the Hurth transmission. So I guess they don't mind running the engine for extended periods of time to charge the batteries. Sabre manual also says to run it full throttle every once in a while to clean things out.

The point of my question was: say a boat does 7 kts with the engine at cruising rpm. Now you are sailing at 7 kts. If you need to run the engine to charge the batteries and you put the engine in gear at cruising rpms (with the boat still under sai) does that put any stress on the drive shaft/transmission or engine?

it would seem silly to run the motor more than say 10 minutes- I find mine charge in far less time but 80 percent of the people you talk to will tell you to keep the engine running for half an hour which is ridiculous
& a waste of fuel

10 minutes will give the batteries a decent charge. However, there are 2 things you should consider, battery health and engine health. Batteries last longer when maintained at full charge. Cylinder walls glaze over on short run times, which results in loss of compression. The engine needs to reach full operating temperatures for a decent amount of time for best health.

I would definitely agree that using the boat engine to charge your batteries is not a good idea.

Wow, this tread is a prime example of why people like me replace sooooo many batteries on boats.

10 minutes does squat for charging a house bank of batteries.... This is simple math & physics.. Ten minutes run time won't even burn off start up condensation in the motor. Here in the cold waters of Maine 10 minutes won't even bring most diesels up to operating temp.

Flooded batteries take 10+ hours to charge to full even with a large charge source because of battery acceptance rates as the bank hits absorption voltage.

A 400Ah bank at 50% state of charge needs roughly 240Ah's returned to it to be full due to charge inefficiency.

With a 100A alternator it will really put out about 80A so roughly 1.3Ah per minute at 80A. Easy math shows that 1.3Ah X 10 minutes is about 13.3Ah returned to the battery or just 5.5% of the energy need to charge this bank to full....

5.5% is a long way from a "decent charge", in 10 minutes....10 minutes of run with a 100A alt on a 400Ah bank barely gets you from 50% SOC to 53% SOC when charge inefficiencies are taken into account..

Also keep in mind that just because a "charger" has gone through bulk, absorption and is into float in 2.5 - 3 hours also means little other than the charger is poorly programmed for the bank you are charging. When a charger comes out of bulk quickly all it usually means is that your batteries are already toast and sulfated enough to build a surface charge. Sulfated batteries can take double to triple the time it takes healthy batteries to charge because the current can't get into the plates. Many "smart chargers" need to be reset or re-started in order to re-enter absorption because the "absorption" stage is "timed", but not to match your bank. Many of these chargers are actually pretty stupid and chronically under charge batteries. Some of them have absorption voltage times of just 1 hour before entering float... Ridiculous really but it is what it is..

Frankly if I am sailing and need to charge the batts I will almost always put her into gear and get a little boost.

On our own boat we have around 3500 hours on the motor and the boat was a 24/7/365 "on the hook" cruiser for 5 straight years. The boat spent perhaps 1500 of those hours charging batteries and running the engine driven refrigeration at anchor. The cylinder walls are NOT glazed, still perfectly cross hatched and she burns no oil and runs as good as new. As long as you run the engine properly when not charging at anchor or under sail these engines can be run like this without much issue at all.

When charging via alternator it is wishful thinking that you'll ever get anywhere close to full unless you spend 10+ hours running the motor like a trawler would.

This is why experienced cruisers use the "cruisers rule" and cycle the bank between 50% SOC and 80-85% SOC. Trying to charge much above 85% SOC is simply a waste of fuel due to declining battery acceptance...

I see no problem with the Hinckley running the motor in that manner. If it were me I would have thrown it into gear and pushed her up her bow wave a tad bit more to keep the prop loaded and keep the engine up on operating temp...

I suspect you meant to say NOT give the batteries a decent charge.
Especially as you put a "like" on Mainesail's post above this.

For the OP, if the ascetics of the boat are the main consideration, considering it's a Hinckley the owner could have gone with a flexible solar panel of 125w or so and simply laid it flat on the deck. Ascetics should never overcome safety, and comfort of the crew is a safety must.

Going to sea in today's age with the tech we have available and expect to be able to use with only one source of power generation is not an option.

If the engine must be run in gear (depends on the transmission type) and the boat is exceeding maximum motoring speed...odds are you are surfing because anything like normal hull speed should be witin engine rpm range.

So I'd run the engine at an appropriate high speed, in or out of gear per the tranny's requirements.

Then there's the idea of running the engine at low speed to charge the batteries. WRONG! Let's say the engine has an idle speed around 800 rpm and a cruising speed around 2600 rpm. If the alternator was properly matched up (pulley size & output curve) then the alternator will not be putting out signficant power at low rpms. At 1000 rpm it may be putting out 20-25 of the rated power, at 2000 spm it may be closer to 85% of the rated power. (Numbers drawn from a hat but good enough for examples.)

You need to know how the system has been designed, or have an ammeter in circuit to see what the alternator is putting out, because they are usually designed to put out something near rated power at something near typical cruising rpm, and all you are doing at lower rpm is wasting fuel.

Part of this is cheap and sloppy design (who notices?) and part of this is money, because an alternator that can put out full power at low rpms costs more. And an alternator that can put out full power at low rpms while also not burning out at sustained high rpms (i.e. trying to claw off a lee shore for eight hours) is more expensive again.

tib- most marine engines are OK at normal heel angles of up to about 15 degrees IIRC.
One thing to consider is the fuel pickup location if you are running the engine while sailing, especially if you have two outboard tanks and only one pickup tube. I made the mistake of starting the engine while heeled over to starboard and sucked air out of the port pickup tube because the fuel had drained through the transfer hose to the starboard tank. I have separate pickup tubes and the tanks are not connected now:-))

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